We have slightly longer stints occasionally but more often than not, DD wakes after 40-45 minutes, crying and needing to be resettled. The only way to encourage longer naps is to hold her while perched on the edge of my bed- kind of doing my back in.
We co sleep (would like to break this cycle but that's another story) and she usually sleeps approx. 7-10pm in cot but after that it's short bursts of sleep again.
I read that babies are able to resettle once they're 4 months. Another mum friend suggested they need to be 'taught' 
Or do I just keep waiting?